Tolerance of plants to adverse growing conditions, including drought, high light intensities, high temperatures, nutrient limitations, saline growing conditions and the like, is a very desired property for crop plants, in view of the never-ending quest to ultimately increase the actual yield of these plants.
Various ways of achieving that goal of improving what is commonly known as the stress resistance or stress tolerance of plants have been described. Since different abiotic stress conditions frequently result in the generation of harmful reactive oxygen species (“ROS”) such as superoxides or hydrogen peroxides, initial attempts to improve stress resistance in plants focused on prevention of the generation of the ROS or the removal thereof. Examples of these approaches are overexpression of ROS scavenging enzymes such as catalases, peroxidases, superoxide dismutases etc. or even increasing the amount of ROS scavenging molecules such as ascorbic acid, glutathione etc. These approaches and other attempts to engineer stress tolerant plants are reviewed e.g. in Wang et al. 2003, Planta 218:1-14.
Stress tolerance in plant cells and plants can also be achieved by reducing the activity or the level of the endogenous poly-ADP-ribose polymerases (ParP) or poly(ADP-ribose) glycohydrolases (ParG) as described in WO00/04173 and PCT/EP2004/003995, respectively. It is thought that in this way, fatal NAD and ATP depletion in plant cells subject to stress conditions, resulting in traumatic cell death, can be avoided or sufficiently postponed for the stressed cells to survive and acclimate to the stress conditions.
Uchimiya et al. (2002) et al. describe the isolation of a rice gene denoted YK1, as well as use of a chimeric YK1 gene to increase the tolerance of transgenic rice plants harboring that gene to rice blast and several abiotic stresses such as NaCl, UV-C, submergence, and hydrogen peroxide. (Uchimiya et al., 2002, Molecular breeding 9: 25-31).
Uchimiya et al. further published a poster abstract describing that overexpression of a NAD dependent reductase gene (YK1) in rice cells also promoted the level of NAD(P)(H) through up-regulating NAD synthetase activities, and concluded that this modification in turn generated a pool of redox substances needed for ROS stress resistance (Uchimiya et al. 2003 Keystone symposium on Plant biology: Functions and control of cell death, Snowbird Utah Apr. 10-15, 2003).
NAD synthetase from yeast has been well characterized and is the last enzyme in both the NAD de novo synthesis pathway and the NAD salvage. In the de novo pathway, quinolate is the precursor for NAD synthesis and is generated as a product of tryptophan degradation. In the salvage pathway, nicotinamide (which is a degradation product of NAD, generated through the action of various enzymes such as PARP, NAD-dependent deacetylases or other NAD glycohydrolases) is the precursor molecule. In a first step, nicotinamide is deamidated to nicotinic acid by a nicotinamidase. The nicotinic acid is transferred to 5-phosphoribosyl-1-pyrophosphate by the enzyme nicotinate phosphoribosyl transferase to yield nicotinic acid mononucleotide. This compound is shared between the de novo and the salvage pathway. Hence, further conversion of this compound by NAD+ pyrophosphorylase and NAD synthetase is achieved as in the de novo pathway.
In yeast, overexpression of PNC1 (encoding nicotinamidase) has been correlated with life span extension by calorie restriction and low-intensity stress (Anderson et al., 2003 Nature 423: p181-185; Gallo et al., 2004, Molecular and Cellular Biology 24: 1301-1312).
WO2004/016726 describes methods and compositions for modulating the life span of eukaryotic and prokaryotic cells and for protecting cells against certain stresses. One method comprises modulating the flux of the NAD+ salvage pathway in the cell, e.g. by modulating the level or activity of one or more proteins selected from the group consisting of PNC1, NMA1, NPT1 and NMA2.
Little is known about the respective enzymes of the NAD biosynthesis pathways in plants. Hunt et al., 2004 describe the use of the available genomic information from Arabidopsis to identify the plant homologues of these enzymes (Hunt et al., 2004, New Phytologist163(1); 31-44). The identified DNA sequences have the following Accession numbers: for nicotinamidase: At5g23220; At5g23230 and At3g16190; for nicotinate phosphoribosyltransferase: At4g36940, At2g23420, for nicotinic acid mononucleotide adenyltransferase: At5g55810 and for NAD synthetase: At1g55090 (all nucleotide sequences are incorporated herein by reference).
PCT/EP 2005/010168 describes methods for increasing the stress resistance in plants and plant cells whereby enzymes involved in the NAD salvage synthesis pathway and/or the NAD de novo synthesis pathway are expressed in plants.
Alternative methods for increasing stress tolerance in plants are still required and the embodiments described hereinafter, including the claims, provide such methods and means.